Tips for increasing oscilloscope vertical resolution - Part I

Do you need your oscilloscope to view increasingly smaller electronic signal detail? You are not alone. Interest is growing in small signal visibilityóboth for current and voltage. Specifically, scope users want better ability to see small signal changes on a large signal (high dynamic range measurements), or seeing small signals where high dynamic range isnít required. Often these signals change over just a few millivolt or milliamps. Industry examples include testing of high-quality power rails, medical technology developed to interacts with human physiology, high-energy physics one-time experiments that produce small pulses, and mobile devices where current and overall power consumption in sleep mode is critical.

Measurement of very small signals can be challenging as the ability to view small signals is impacted not only by the noise of the scope, but also by scope settings and probing. The following seven techniques can help your scope see smaller signals than youíve previously seen with it.

Tip 1: Start with a scope that has low noise

While the other techniques described to see small signals are scope vendor agnostic, having a scope with low noise is critical if you really want visibility to small currents and voltages. You wonít be able to see signal details smaller than the noise level of the scope.

Whatís a quick way to check how much noise a specific scope has? Most oscilloscope vendors will characterize noise for a specific model numbers and include these values on the product datasheet. If not, you can ask for the information, or find out yourself. Itís easy to measure in a few minutes. Disconnect all inputs from the front of the scope and set the scope to 50 ? input path. You can also run the test for the 1M? path. Turn on a decent amount of acquisition memory, 100Kpts to 1Mpts will suffice, run the scope with infinite persistence and see how thick the resulting waveform is. The thicker the waveform, the more noise the scope is producing internally.

Each scope channel will have unique noise qualities at each vertical setting. You can view the noise visually just by looking at wave shape thickness, or you can be more analytical and take a Vrms AC measurement to quantify. Create a chart as the one shown in figure 1. These measurements will allow you to know how much noise each scope produces. Donít expect to measure signals that are less than the noise of the scope.

The industry now offers several scopes with more than 8-bits of resolution. How valuable are the additional bits? Provided there is a sufficient signal-to-noise ratio (SNR), more ADC bits allow finer details of the signal to be seen. Noise typically plays a greater role in limiting some of the effectiveness of the additional bits of resolution.

Figure 1: Noise is typically characterized with typical values listed in vendor datasheets. Alternatively, you can characterize noise on your scope in a few minutesClick on image to enlarge